1. Field of the Invention
The invention relates to magnetic resonance (MR) devices such as magnetic resonance imagers and spectrometers, and more particularly, to a method and apparatus for substantially reducing disturbances in pulsed magnetic field gradients produced in these devices.
2. Description of the Prior Art
MR systems are based on the phenomenon of nuclear magnetic resonance (NMR). When an object is placed in a magnetic field, the field causes the spin vectors of certan types of nuclei (e.g. .sup.1 H, .sup.13 C, .sup.31 P and .sup.23 Na) to orient themselves with respect to the applied field. The nuclear vectors, when supplied with the right amount of energy, will reorient themselves in the field and emit or absorb energy in the process. The energy needed to perturb the nuclear spin vectors is in the radio frequency range, and the specific frequency depends upon the strength of the magnetic field which is experienced by the nuclei. In MR devices which do not provide electrical control of the spatial positioning of the applied magnetic fields, the sample is placed in a large, uniform, static magnetic field. The sample is perturbed by a pulse of radio frequency energy, and the frequency response signals of the perturbation are recorded. A measure of signal intensity as a function of resonance frequency or magnetic field at the nucleus is obtained and analyzed, in a manner well known, to derive an image or spectroscopic information about the sample.
Imaging and spatially dependent spectroscopic analysis methods carrying the technique one step further by using magnetic field gradient in addition to a primary background (main) uniform magnetic field. Since the resonance frequency of the nuclei depends upon the precise magnetic field strength imposed upon it, applied field gradients are used to provide a technique for spatial encoding. MR devices correlate signal intensity at a given frequency with sample concentration and relaxation parameters as a given location. This provides spatial information which is used to make a map or image of the object, based upon signal intensity variations due to concentration and/or relaxation time differences. In a spectrometer, these field gradients allow a spatial selection of a particular portion of the sample object to be analyzed. The field gradients are produced with a set of gradient coils. These coils are often referred to as "pulsed gradient coils" because they are energized by pulses which grade the main field in two or more orthogonal directions.
Imaging the entire body of a patient, for example, typically requires a steady, high homogenity, main field and highly linear gradients in the range of, for example, 0.1-1.0 gauss/cm with rise and fall times as short as possible, typically on the order of 0.1 to 1.0 milliseconds. An axial gradiant (i.e. in the "Z" direction) is typically produced by solenoid coils while radial gradients (which define "X" and "Y" coordinates) are formed by saddle-shaped coils, as well known.
Regardless of the way in which the background field is produced, for example by a superconducting magnet system, the changing magnetic fields which result from the pulsing of the gradient coils will induce eddy currents in any nearby conducting media (such as radiation shields and cryogen vessels included in a superconducting magnet system). These eddy currents have an adverse effect on both the spatial and temporal quality of the desired gradient fields. The eddy currents themselves generate a field which superimposes on the field produced by the gradient coils thereby disturbing the gradient coil field from its desired level and quality, in both space and time. The result of this perturbation is that the amplitude and phase characteristics of the MR signals are distorted, thereby reducing the accuracy of the spectroscopic analysis or the quality of the generated images. Therefore, it is necessary that the eddy currents be carefully controlled, compensated for or reduced to an insignificant level.
As known those familiar with MR devices, the effects of induced eddy currents can be classified into magnetic effects within the gradient field which are position dependent (dependent upon the particular position of the sample within the gradient field) and those which are position independent. An object of the present invention is to substantially reduce the position independent field effects caused by eddy currents.
Although one might think that careful manufacturing could result in a balancing out of these eddy currents effects, manufacturing tolerances of the large components of MR devices, in particular the positioning of the radiation shields, exceeds the accuracy needed for the successful implementation of advanced spectroscopy and imaging techniques.
An existing solution to this eddy current problem is to provide a self-shielded gradient coil which, in concept, prevents the gradient field from penetrating to the surrounding structure of the main magnet. U.S. Pat. No. 4,733,189 issued Mar. 22, 1988 is illustrative of this technique and discloses an active shield. This approach suffers from several drawbacks. Firstly, the diameter of the gradient coil is reduced due to the presence of the active shield. This limits the size of the objects which can be investigated. Secondly, the power consumption of the gradient coil is increased due to the close proximity of the active shield to the gradient coils. Furthermore, any eccentricity between the active shield and the gradient coils produces a base field shift during application of the gradient pulse.
It is a further object of the present invention to provide a method and apparatus for substantially reducing the effects of position independent field disturbances caused by eddy currents in a manner which avoids the above-mentioned disadvantages of an active shield.